The use of powder metallurgy (P/M), and particularly iron and iron alloy powders, is known for forming magnets, including soft magnetic cores for transformers, inductors, AC and DC motors, generators, and relays. An advantage to using powdered metals is that forming operations, such as compression molding, injection molding and sintering techniques, can be used to form intricate molded part configurations, such as magnetic cores, without the need to perform additional machining and piercing operations. As a result, the formed part is often substantially ready for use immediately after the forming operation.
Molded magnetic cores for AC applications generally should have low magnetic core losses, which requires that the individual metal particles within the magnetic core be electrically insulated from each other to provide eddy current protection, while also achieving an acceptable level of permeability. Numerous types of insulating materials have been suggested by the prior art, many of which also serve as a binder that adheres the particles together. Examples of such materials include inorganic materials such as iron phosphate, alkali metal silicates, and organic polymeric materials. In addition to providing adequate insulation and adhesion between the metal particles upon molding, insulating materials are often selected for their ability to provide sufficient lubrication during the forming operation to enhance the flowability and compressibility of the particles, and therefore enable the particles to attain maximum density and strength, particularly when compression molded at high pressures.
In view of the above considerations, plastics have been widely used as insulating materials for AC magnetic cores. However, the permeability of magnetic articles formed with plastic insulating materials is not sufficiently high for many AC applications, and core losses are often high at low frequencies (e.g., 50 Hz and less), resulting in low outputs at low rpms. Increased permeability and lower hysteresis losses can be achieved by annealing the core to relieve the detrimental effects on magnetic characteristics caused by cold working during compression molding. However, relieving substantially all stresses in a work-hardened core formed of ferromagnetic materials often requires maintaining the core at a temperature of at least 600.degree. C. for a length of time that depends on the degree of work hardening in the core, followed by slow cooling. Plastic materials currently available are unable to withstand these temperatures, and degrade and pyrolyze during annealing. The ability of the insulating material to encapsulate and adhere the particles will also degrade if the core is annealed at lower temperatures that exceed the heat deflection temperature of the insulating material. Even if physical destruction of the core does not occur, the magnetic field characteristics of the core will likely be severely impaired because of the degradation of the insulating capability of the material.
In view of the above, it can be appreciated that, because the insulating material must remain within an AC magnetic core to achieve low core losses, the ability to anneal a core is limited by the heat resistant properties of the insulating material. Maximum operating temperatures of AC magnetic cores are similarly limited by the insulating material. Therefore, it would be desirable to provide a coating for powdered metals that has the ability to withstand high processing and operating temperatures, so that P/M magnetic cores molded from such particles exhibit desirable mechanical and magnetic properties that do not deteriorate at high temperatures.